What is thimerosal? The rarely-used vaccine preservative targeted by RFK Jr

Independent

18 hours ago

A federal vaccine panel, recently reshaped by the U.S. Health Secretary Robert F. Kennedy Jr. has voted to discourage the use of flu vaccines containing thimerosal, a mercury-based preservative. The decision marks a dramatic shift in vaccine policy, as thimerosal has long been considered safe by health agencies worldwide, with its use already limited to a few multi-dose flu shots.
RFK Jr. has long linked thimerosal to autism – a connection that extensive scientific research has thoroughly debunked.
Thimerosal is an organic chemical containing mercury, used as a preservative in vaccines since the 1930s. Its effect comes from the mercury that disrupts the function of enzymes in microbes, such as bacteria and fungi. This prevents contamination of vaccines while they are stored in vials. Mercury, however, is also well-known as a potent toxin acting on cells in the brain.
Much of mercury's toxicity to brain cells stems from the same attributes that make thimerosal such a useful preservative. It disrupts the basic biological function of cells by changing the structure of proteins and enzymes.
In the brain, this can lead neurons to become excessively active, can impair the way they use energy, it can increase inflammation and lead to the death of neurons. While mercury poisoning can damage brain function in adults, babies are even more vulnerable.
People have long understood that mercury is toxic. But in the latter half of the 20th century, scientists discovered that industrial mercury entered rivers and seas, accumulating in the tissues of fish and shellfish. The neurological consequences of consuming too much contaminated seafood could be severe. This led environmental scientists to determine safe levels of mercury exposure.
Anxiety about mercury in vaccines intensified when it was noticed that some children receiving multiple vaccines could exceed established safety limits for mercury exposure. These limits were based on environmental toxicity studies. How mercury affects the brain, though, depends very much on the chemical form in which it is ingested.
Methylmercury v ethylmercury
The form of mercury that contaminates the environment as a consequence of industrial processes is methylmercury. The form that is part of thimerosal is ethylmercury.
The structure of these molecules differs in subtle but important ways. Methylmercury has one more carbon atom and two more hydrogen atoms than ethylmercury. These small differences significantly affect how each compound behaves in the body, particularly in how easily they dissolve in fats.
Fat solubility is a key consideration in pharmacokinetics – the science of how drugs and other molecules travel through the body. Briefly, because cell membranes are made of fatty substances, a molecule's ability to dissolve in fats strongly influences how it crosses these membranes and moves through the body.
It affects how a molecule is absorbed into the blood, how it is distributed to different tissues, how it is broken down by the body into other chemicals and how it is excreted.
Methylmercury from environmental contamination is more fat-soluble than ethylmercury from thimerosal. This means that it accumulates more easily in tissues and is excreted from the body more slowly.
It also means that it can more easily cross into the brain and accumulate at greater concentrations for longer. For this reason, the safety guidelines that were established for methylmercury were unlikely to accurately predict the safety of ethylmercury.
Global policy shift amid public fear
Nevertheless, concerns about vaccine hesitancy, rising autism diagnoses and fears of a potential link to childhood vaccines led to thimerosal being almost entirely removed from childhood vaccines in the US by 2001 and in the UK between 2003 and 2005.
Beyond biological considerations, policymakers were also responding to concerns about how vaccine fears could undermine immunisation efforts and fuel the spread of infectious diseases.
Denmark, which removed thimerosal from childhood vaccines in 1992, provided an early opportunity to study the issue. Researchers compared the rates of autism before and after thimerosal's removal, as well as compared with similar countries still using it. Several large studies demonstrated conclusively that thimerosal was not causing autism or neurodevelopmental harm.
Despite the overwhelming evidence that thimerosal is safe, it is no longer widely used in childhood vaccines in high-income countries, replaced by preservative-free vaccines, which must be stored as a single dose per vial.
Storing multiple doses of a vaccine in the same vial, however, is still an extremely useful approach in resource-limited settings, in pandemics and where diseases require rapid, large-scale vaccination campaigns, common with influenza.
International health bodies, including the World Health Organization, continue to support thimerosal's use. They emphasise that the benefits of immunisation far outweigh the theoretical risks from low-dose ethylmercury exposure.